1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus and a method for manufacturing an exhaust gas purifying apparatus.
2. Discussion of the Background
Particulate matters (hereinafter, also referred to as PMs) are contained in exhaust gases discharged from internal combustion engines such as diesel engines, and in recent years, there has arisen a serious problem that these PMs are harmful to the environment and the human bodies. Moreover, since exhaust gases also contain toxic gas components such as CO, HC, and NOx, there have been growing concerns about influences of these toxic gas components on the environment and the human bodies.
In view of these, as an exhaust gas purifying apparatus for capturing PMs in exhaust gases and for purifying the toxic gas components, various exhaust gas purifying apparatuses have been proposed. Each of the exhaust gas purifying apparatuses is configured by: an exhaust gas treating body including a porous ceramic, such as silicon carbide and cordierite; a casing for housing the exhaust gas treating body; and a holding sealing material including an inorganic fiber aggregated body that is disposed between the exhaust gas treating body and the casing.
This holding sealing material is installed mainly for purposes of preventing the exhaust gas treating body from being damaged upon contact with the casing that covers its periphery because of vibrations and impacts caused by traveling or the like of an automobile and for preventing exhaust gases from leaking between the exhaust gas treating body and the casing.
Here, since the internal combustion engines are operated at an air fuel ratio close to the theoretical air fuel ratio for the purpose of improvement in fuel consumption, exhaust gases tend to increase their temperature and pressure. When exhaust gases having a high temperature and a high pressure reach the exhaust gas purifying apparatus, the difference of coefficient of thermal expansion between the exhaust gas treating body and the casing leads to variations of the interval therebetween. Therefore, the holding force of the holding sealing material that does not change the location of the holding sealing material even with some variations of the intervals is required.
In order to meet such demands, an expansive holding sealing material has been used. The expansive holding sealing material includes inorganic fibers and an expansive agent that is in a small size when the expansive holding sealing material is installed in a casing and expands by heating to a high temperature. The expansion of the expansive agent at a high temperature is used to improve the holding force of the holding sealing material.
However, the amount of the expansive agent in an expansive holding sealing material is limited because too-much expansive agents may collapse an exhaust gas treating body in the expansion. In addition, with an aim of overcoming the low holding force at a low temperature, it is necessary to apply heat to the holding sealing material in advance of the actual installation thereof to an exhaust gas purifying apparatus. Or alternatively, it is necessary to use a clamp in a metal casing to mechanically prevent a drop of a holding sealing material.
On the other hand, in an unexpansive holding sealing material including unexpansive inorganic fibers, the repulsive force of the inorganic fibers, which generates the holding force, is secured by increasing the weight per unit area of the holding sealing material. As a result, the pressure per unit area (hereinafter, also referred to as surface pressure), which is to be applied to the holding surface of the holding sealing material, is increased to improve the holding force of the holding sealing material.
However, in this method, since the holding sealing material is unexpansive, thermal expansion of a metal casing due to high-temperature exhaust gases may result in the lowered surface pressure of the holding sealing material, namely, the lowered holding force of the holding sealing material. In addition, increasing the weight per unit area of the holding sealing material in expectation of the thermal expansion of the metal casing requires a large amount of inorganic fibers, which is uneconomical. This tendency is more likely to be found in a large exhaust gas purifying apparatus for handling the exhaust gas treatment of a large internal combustion engine. Further, development of a holding sealing material which exerts the sufficient holding force in the minimal amount has also been demanded in economic standpoint.
With an aim of meeting the above demand, there has been proposed an exhaust gas purifying apparatus including a holding sealing material having a similar weight as the conventional one and an anti-drop mechanism for preventing a drop of the exhaust gas treating body in the metal casing, instead of increasing the weight per unit area of the holding sealing material.
As such an exhaust gas purifying apparatus, Japanese Patent Application Publication (KOKAI) No. 2002-97945 discloses a catalyst converter including a storage part in which a recessed bead is formed and a monolith type catalyst carrier is stored.
The contents of Japanese Patent Application Publication (KOKAI) No. 2002-97945 are incorporated herein by reference in their entirety.